1. Field of the Invention
The present invention relates to optical beam steerers of the type employed in optical scanning and tracking systems, and more particularly to such beam steerers which utilize a mechanically-positioned beam deflecting mirror.
2. Description of the Prior Art
Before discussing the prior art, it is noted that reference is made below to FIGS. 1A through 1C which illustrate certain prior art beam steering arrangements:
An essential element in a variety of information processing systems of the type which include optical scanning and/or tracking apparatus is a controllably positionable light beam deflecting mirror. In high speed scanning or tracking systems, for example those used in optical computer output devices, the beam deflecting mirror must be capable of rapidly and accurately directing an incident light beam to a desired location.
In comparing and evaluating the performance of beam steerers, various characterizing parameters are useful. One such parameter is system bandwidth, which serves to indicate how rapidly the beam steerer is capable of responding to a control signal. In this regard, it is noted that the dynamic properties of mirror-type beam steerers tend to limit operation to scanning frequencies in the acoustical range and generally to scanning frequencies below several kilohertz. This limitation is the result of the attenuation which occurs when the mirror system is operated above the lowest mechanical resonance frequency for the system. This resonance frequency is determined by certain physical characteristics of the system, such as the moment of inertia of the mirror and the stiffness of the system driving the mirror. Generally, an acceptable bandwidth for scanning or tracking purposes is achieveable by appropriate selection of steering system components and, accordingly, bandwidth limitations have not proven a serious deterrent to the use of mirror-type beam steerers in optical information processing apparatus.
A further important characterizing parameter is the number of resolvable positions (which in the art are called "spots") which a beam may be caused to assume. (This number is often referred to as resolution or resolving power of the system.) System resolution provides an indication of the amount of information which can be scanned with each sweep of the steerer. Mirror size and the maximum range of angular deflection for the mirror both influence resolution. It should be noted, however, that, while the number of spots is increased by increasing mirror size, the mirror's moment of inertia is attendantly increased with the result of reducing steerer bandwidth.
The maximum range of angular deflection (sweep range) is, moreover, of considerable interest in its own right as a characterizing parameter. By increasing the sweep range of a system, the length of a scan trajectory can, for example, be increased. A significant benefit of a large sweep range, it follows, is that a large area of, for example, a record medium, can be scanned with less movement of that medium in relationship to the steerer.
A further important characterizing parameter is the product obtained by multiplying resolution and bandwidth. Resolution-bandwidth product serves to indicate the rate at which information can be transferred (a higher resolution-bandwidth product indicates a potentially higher information transfer rate). This is because resolution-bandwidth product is representative of not only the number of resolvable information positions for a single scan but also how rapidly the beam can sweep over those positions.
Proceeding from the foregoing brief overview, it is helpful to consider some of the various mirror-type, beam steering systems which have been developed.
One type of steering system is electromagnetically driven and is commonly referred to as a galvanometer scanner. Systems of this type often rate high in performance (resolution-bandwidth product and sweep range), but are expensive, complex, and can present hysteresis problems (see E. P. Grenda et al. "Closing the Loop on Galvo Scanners", Electro Optical Design, pages 32 through 34, April, 1974).
Several others of the known types of beam steering systems utilize mirrors which are directly attached to piezo-electric shear transducers which act as drivers (see: J. J. Shaffer et al. "Bender-Bimorph Scanner Analysis", Applied Optics, pages 933 through 937, April, 1970; U.S. Pat. No. 3,544,201; U.S. Pat. No. 3,794,410; and U.S. Pat. No. 1,438,974). The transducer drivers are often referred to as "bimorphs" or "bimorph benders". FIGS. 1A, 1B, and 1C of the drawings illustrate the configurations of three types of steering systems using bimorph strips as drivers. In these figures, elements 2a, 2b, and 2c are mirrors, and elements 4a, 4b, and 4c are bimorphs which serve as drivers. Such systems generally provide a high resolution-bandwidth product, are simple in construction, and are low in cost. Because of these desirable characteristics, bimorph-driven steerer systems have achieved general acceptance, particularly in optical information processing equipment.
One shortcoming of bimorph-driven steerer systems, however, arises because bimorph-benders, while providing forces that are generally more than adequate, do so over very small deflection and tilt angle ranges: 0.004 inches deflection and 1/2.degree. tilt angle are approximate numbers for a one-inch long bimorph-bender. Larger deflections would be desirable and, to overcome this deficiency, beam steerers aree often cascaded to provide an increased range of beam deflection angles and to improve system resolution (see V. J. Fowler et al. "A Survey of Beam Deflection Techniques", Applied Optics, pages 1675 through 1682, October, 1966; and U.S. Pat. No. 3,544,201). With this approach, attenuation of the beam is increased as a result of the additional reflections, and the cost and complexity of the overall apparatus increases markedly because the cascaded beam steerers must be synchronized to achieve the desired cumulative beam deflection.
A "compounding" of this complexity occurs where two-dimensional beam steering is desired. In such prior art situations, two sets of cascaded beam steerers are utilized, each of which provides a cumulative deflection of the beam in a different direction. It will be appreciated that such equipment duplication results in high equipment costs, and greatly intensifies the problems of mirror synchronization and alignment.